ABSTRACT: An age-related impairment of the regenerative capacity of aged muscle is a major contributor to declines in functional mobility and is associated with an increased morbidity in an elderly population. Following an acute injury, young skeletal muscle initiates a highly effective regenerative response, which largely restores the original architecture of the damaged fibers. Conversely, with increasing age, the regenerative response to injury results in a considerable scar tissue deposition at the expense of functional contractile tissue. Much of this healing defect has been attributed to an age-related decrease in muscle stem, or satellite, cell (MuSC) functionality. In response to skeletal muscle injury, MuSCs become activated from a quiescent state to repair damaged myofibers. However, it has been suggested that the increased fibrosis deposition following injury is a result of a myogenic-to-fibrogenic conversion and cellular senescence of MuSCs. Fortunately, these age- related changes are reversible. Elegant studies employing heterochronic parabiosis, in which the circulatory systems of young and aged animals are conjoined, have revealed that rejuvenation of the systemic microenvironment significantly restores both whole tissue and MuSC regenerative capacity in aged muscle. These findings implicate that circulating factors, such as Klotho, play a critical role in dictating skeletal muscle regenerative potential over time. Elucidation of the origin and nature of circulating factors contributing to the aged muscle phenotype is critical for the development of strategies to prevent, delay or reverse age-related declines. Consistent with the objective of the FOA, the overarching goal of this study is to identify a novel role for the anti-geronic protein, Klotho, in mediating declines in muscle healing capacity with increasing age, and to mechanistically test our hypothesis that age-related declines in neuromuscular activity contribute to attenuated Klotho expression. Specifically, in Aim 1, we will interrogate the underlying mechanism by which Klotho expression regulates MuSC mitochondrial function and skeletal muscle regenerative potential. In Aim 2, we will define the molecular mechanisms by which contractile activity stimulates Klotho expression and promotes muscle healing. These studies, when completed, will have a long and lasting impact on the field as they will establish Klotho as an important anti-geronic factor that regulates MuSC activity essential for functional muscle regeneration after injury. In addition, these experiments will lay the groundwork for future studies in which muscle stimulation in geriatric populations may be used to prevent, delay or reverse age-related declines in muscle function through improved regenerative capacity.